1. Field of the Invention
This invention relates generally to the field of turbochargers for internal combustion engines and more particularly to a turbocharger turbine volute having a flexible dividing wall defining and controlling the throat section of the volute at the end of the flexible wall with the wall movable to alter the end position, thus reducing the area and percentage of the periphery of the volute that is discharging flow into the turbine wheel.
2. Description of the Related Art
Turbochargers are employed on numerous forms of internal combustion engines for use in automobiles and other vehicles. Turbochargers typically employ a radial turbine operating with an inlet volute supplying exhaust gas from the engine to power a compressor for inlet charge to the engine. Controlling the turbine to achieve desired power in the turbocharger is often required. The purpose of a variable geometry turbine is to be able to actuate some movable mechanism such that it results in a controllable variable turbine power output. On the corrected flow curve shown in FIG. 1, assuming a constant physical flow, closing the vanes of a conventional variable nozzle turbine moves the operating point to a lower corrected flow and a higher expansion ratio, thus producing more power.
The problems and limitations of variable geometry turbines are well known and there is a continual search for a better mechanism to overcome these problems. Turbines with fixed and variable nozzle vanes are the standard in many industrial applications—aero gas turbines, industrial gas turbines, turbo-expanders, steam turbines, etc. Fixed nozzle vanes have not traditionally been standard in turbochargers due to narrow operating range, vibration-induced fatigue failures of the turbine wheels, and noise generated by the turbine blades passing the nozzle vanes.
The reason for the success of nozzles in traditional applications and the relative lack of success in turbochargers is that turbocharger turbines operate over an extremely wide speed range, generally at least 10/1. In most industrial turbine applications, the turbine speed range is usually very limited (˜2 or 3/1). Resonances between the vibration-inducing nozzles and the natural vibrational modes of the turbine wheel can be tolerated in most industrial applications if the turbine operates in resonance for only a few seconds during start-up and shut-down. The wide speed range of the turbocharger makes it exceptionally difficult for the designer to push these resonances either above the maximum speed or below the band of normal operation.
As the vanes in a conventional vaned variable geometry turbine are closed down, the expansion through the vanes reaches the critical point where the flow goes supersonic and shocks are established downstream. As each passage creates its own shock (or multiple shocks) each turbine blade cuts through hundreds or thousands of shocks per second. If the frequency of this shock cutting coincides with a vibrational mode of the turbine wheel, the turbine wheel can fail in just a few minutes. The turbine wheel has many blade and hub modes, and orders of vibration must be accounted for as well so it is quite a complex problem. Even “clusters” of computers running the most sophisticated computational fluid dynamics linked with finite element stress and vibration analysis with auto-optimization routines have difficulty converging on a solution.
As the need for more control over the boost and back-pressure of internal combustion engines has increased, variable geometry turbines have become prevalent in modern engines. Unfortunately, this has resulted in many field issues due to blade vibration failures and has restricted the design latitude for the turbine designer such that there may not be a solution or the solution has severe consequences—poor aerodynamics or high inertia.
While a number of variable geometry turbines have been invented that have no nozzle vanes, they are not commonly used since the performance over the complete operating range is usually lacking. The performance of the variable geometry turbine in a turbocharger is of utmost importance to the engine designer. Turbochargers have a well-known characteristic of poor performance at low engine speeds and of delay or lag in responding to up-power transients. The inertia of the rotor group is commonly identified as the responsible design element. However, the major contributor is the efficiency of the variable geometry turbine at low engine speeds with the vanes operated quite far closed.
FIG. 2, reconstructed from Neil Watson and Marian Janota's, “Turbocharging the Internal Combustion Engine” published by The MacMillan Press Ltd. 1982 shows the classic graph of radial turbine efficiency versus the ratio of the turbine speed U and the isoentropic gas velocity C. Fundamentally, this graph shows the limitations of a radial turbine operating at low engine speed. The U/Co is quite low in this operating condition and when an up-power engine transient is executed, the vanes are closed further. This drives up the gas velocity while the turbine speed remains low. The U/Co parameter is often driven down to 0.3 in steady state or below 0.2 in transient operation which results in extremely low turbine efficiency and thus poor response.
A massive amount of work by turbocharger engineers over the years has gone into trying to fix this issue. Unfortunately, it is controlled by the basic physics of the radial turbine. Therefore, another objective of this variable geometry turbine design is to find a “loophole” in the basic physics of the radial turbine to improve the efficiency of the turbine at low blade speed ratios.
FIG. 3 is a generic graph which shows the typical characteristics a vaned variable geometry turbine. Observing the characteristics of the efficiency islands, one can notice that the peak efficiency occurs at a fixed corrected flow. An inference can be drawn from this data that high efficiency is achieved when each blade passage has an ideal corrected flow rate. To achieve high efficiency at low flow rates, an obvious solution would be to use a smaller turbine. Making a variable-sized turbine wheel is impractical as the turbine wheel rotates at up to 500 meters per second tip speed.
It is therefore desirable to eliminate failures of turbine wheels due to vibration induced from the variable mechanism. It is further desirable to enhance the performance of the turbine at low speed-to-gas-velocity (U/Co) ratios. It is additionally desirable to reduce noise as a corollary effect of reducing the turbine wheel failures.
Many fixed geometry turbochargers use divided turbine housings where the exhaust from the cylinders is ducted into two (or more) passages and the exhaust from these groups is kept separate until the turbine wheel entrance. The purpose of this is to take advantage of “pulse charging” and to improve the scavenging of cylinders by preventing high pressure pulses traveling upstream in adjacent cylinders. Conventional variable geometry turbines cannot take advantage of “pulse charging” as the restriction of the vanes create an upstream backpressure which reduces the pulse and geometrical limitations prevent meriodonal separation of the flows through the vanes. It is therefore further desirable to remove the limitation on effectively using “pulse charging” with a variable geometry turbine.